Soil phosphorus concentrations croplands and forage lands in the Central United States are frequently at 100% fertility index or greater, a feature attributed to fertilization and manure applications. [19] determined phosphorus inherited from natural soils that were subsequently converted to agriculture and the inevitable imposition of farming practices were the main influencers of cropland soil P variability, but the magnitude of their contribution was predicated on the total soil P and labile inorganic P. [20] estimated a global averaged inorganic labile P at 187 kg∙P∙ha⁻¹ for cropland and 91 kg∙P∙ha⁻¹ for grassland, but the values vary based on mineralization rates. In humid regions, [22] noted that sulfate is the sulfur source for immediate plant uptake; however, sulfate only contributes up to 5% of total sulfur pool. In most mid-western soils, sulfur covalently bonded in soil organic materials is approximately 95% of total sulfur pool. Sulfur content varies widely, given the presence of gypsum or pyrite and sulfur atmospheric deposition.

Much has been published concerning soil phosphorus ([11]). Recently, [6] noted the major influencers of phosphorus bioavailability include: 1) edaphic factors (soil pH, clay contents, Fe oxyhydroxides), 2) Climate (temperature and precipitation), 3) (weathering and parent material), 4) biological cycling (root exudates and phosphatase enzymes mineralize organic P compounds (inositol hexakisphosphates) into bioavailable phosphate). Soil sulfur chemistry has been reviewed by [23].

The Mississippi River receives and provides water to aquifers underlying its floodplains, with aquifer gains during large river discharge events and aquifer declines during baseflow intervals. [27]) demonstrated that dissolved phosphorus concentrations in the Mississippi River valley alluvial aquifer are considered elevated and range from 0.12 to 1.2 mg∙L⁻¹. They documented that the dissolved iron concentrations increase with reducing conditions and subsequently facilitate phosphorus transport. More recently, [9] reported that groundwater in the Mississippi Delta had some of the highest phosphorus concentrations and widespread reducing conditions in shallow groundwater and long-term P fertilization are important parameters.

Analyzing Mississippi River water and its associated sediments, [29] observed that during significant flooding events soluble reactive P, dissolved organic P, Fe, and Mn concentrations increased because of decreased sediment redox potential. [2] noted that watersheds dominated by urban and cultivated cropland frequently had elevated phosphorus concentrations.

Phosphorus legacy is a soil condition that occurs when soils have had a history of phosphorus fertilization that intensifies the phosphorus bioavailability and predisposes phosphorus overland and subsurface flow to freshwater resources. In the Lower Mississippi Alluvial Plain, [28] documented that phosphorus legacy provided moderate levels of available P and that soil phosphorus concentrations are higher than expected when considering currently reduced phosphorus fertilizer application rates. The data supports the premise that naturally high levels of soil phosphorus exist across the region’s soils.

Water quality degradation, because of legacy soil phosphorus export, frequently results in water resource eutrophication. [5] proposed to utilize the molar ratio of extractable P/(Al + Fe), called the soil phosphorus saturation ratio, to estimate whether soils in humid climates may act as either phosphorus sinks or sources. The soil phosphorus saturation ratio was an effective threshold value, above which water-soluble phosphorus abruptly increases. The threshold phosphorus saturation ratio of 0.10 appears appropriate for acidic, humid-region soils. [5] further proposed that the soil phosphorus saturation ratio in acid soils has the potential of estimating legacy P. Using studies conducted in Europe and the southeastern United States, [14] determined that when soils are below a threshold phosphorus saturation ratio value, the equilibrium phosphorus concentration is low, and any subsequent phosphorus export would be of low to moderate intensity.

With the status of freshwater and marine eutrophication, phosphorus transport mitigation to water resources is an area of intense research ([18]). Land management and soil engineering are leading stewardship activities to limit nutrient migration from agricultural fields to freshwater resources. [3] listed and discussed important farming practices that address soil management of legacy phosphorus, and the land management practices include 1) modern nutrient stewardship to reduce nutrient runoff, 2) appropriate manure application rates, and 3) development and implementation of a well-designed phosphorus targeted drawdown program. In accessing legacy phosphorus, [7] proposed using cover crops, organic acid amendments, or phosphorus-solubilizing microbes to convert inert phosphorus into bioavailable phosphorus.

[24] proposed agricultural management techniques to reduce the incidence of soil legacy phosphorus, including liming acid soils, appropriate crop rotations, intercropping, cover crops, modern soil fertility programming, efficient crop varieties, and inoculation with P-solubilizing microorganisms. In Canada, [4] determined that riparian wetlands can potentially intercept phosphorus mobilized from upland soils before it reaches connecting waterways. [26] documented that appropriately designed reservoirs support phosphorus retention of P. In the Mississippi Alluvial Valley, [1] noted that conservation practices such as conservation tillage, cover crops, and variable-rate fertilization are essential for mitigating agricultural runoff. However, runoff concentrations are more heavily influenced by extreme precipitation events than conservation activities. [15] addressed water quality degradation attributed to erosion, runoff, or leaching. They proposed that land management practices must be implemented as a system of practices which increase redundancy and consider multiple phosphorus transport pathways. Implementation planning must initially consider the entire watershed to guarantee land practices are appropriately placed in critical source areas.

The objectives of this study are: 1) to briefly review phosphorus and sulfur soil fertility in the Mississippi River Basin, with an emphasis on phosphorus as a potential eutrophication species, and 2) to document the sulfur and phosphorus accumulation in several annually inundated floodplain soils of the Mississippi River in southeastern Missouri.

The study area is in east-central Missouri (Cape Girardeau County and Scott County) and residing in annual floodplain regions of the Mississippi River. The dominant forest classification is a temperate mesic forest, frequently a habitat having moderate, well-balanced moisture levels, deep and nutrient-rich soils, with a closed canopy of deciduous or mixed trees. Common forest species within the Mississippi River floodplain include eastern cottonwood (Populus deltoides), white oak (Quercus alba), yellow poplar (Liriodendron tulipifera), American sycamore (Platanus occidentalis), green ash (Fraxinus pennsylvanica), sweetgum (Liquidambar styraciflua), and bald cypress (Taxodium distichum) and others. The climate is humid continental, and the annual rainfall is 1.07 m and average daily annual air temperature is 14˚C ([8]). Annual flooding durations are generally 1 to 7 days; however, long flood intervals may occur. The modern Mississippi River floodplain soils are Holocene age and include the soil orders Entisols, Inceptisols, Mollisols, and Vertisols.

Two pedons of the Commerce series (Fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquepts) and two pedons of the Caruthersville series (Coarse-silty, mixed, superactive, calcareous, thermic Typic Udifluvents) were morphologically described and sampled. The vegetation at the Caruthersville site consisted of Giant River Cane (Arundinaria gigantea). The Commerce pedons presented A-Bw-Bg-Bssg horizon sequences and displayed ochric and cambic horizons, whereas the Caruthersville pedons presented A-C-Cg horizon sequences and displayed ochric epipedons. The Commerce site vegetation consisted of multiple tree species typical of a temperate mesic forest.

The Commerce pedons were located at Universal Transverse Mercator latitude, 277755, 4133200, 16S, whereas the Caruthersville pedons were located at Universal Transverse Mercator latitude, 283178, 4115480, 16S. At each site, soil samples were collected at 15, 30, 46, 61, 91 cm depths, with the Caruthersville also sampled at 122 cm. At each site, the pedon spacing was approximately 15 meters.

Soil pH in water, exchangeable cations, total neutralizable acidity, soil organic matter content by loss on ignition, Bray-1 phosphorus, and 2M potassium chloride extractable SO₄-S were determined by the University Missouri-Columbia Soil Testing Laboratory. For slightly alkaline soils, the Bray-1 phosphorus test may not fully characterize the phosphorus content as representing plant-available phosphorus. In Missouri, an experimentally defined empirical formula based on the actual soil test value and the Bray-1 P desired value (22.5 mg∙P∙kg⁻¹) estimates the intensity of surplus or deficient phosphorus for row-crop and forage fertilizer recommendations. If the actual soil test and desired soil test values are equal, the soil is said to be at 100% soil test level or 100% soil phosphorus index. The Bray-1 phosphorus soil test levels vary with crop selection, with the typical ideal soil test value for row-crops established as 22.5 mg∙P∙kg⁻¹. Typical SO₄-S soil test values are: 1) very low (0 - 4 mg∙kg⁻¹), 2) low (4 - 7 mg∙kg⁻¹), 3) medium (8 - 11 mg∙kg⁻¹), 4) high (12 - 15 mg∙kg⁻¹), and 5) very high (16+ mg∙kg⁻¹) ([10]).

An aqua-regia digestion was performed to estimate near-total elemental concentrations. In this procedure, 0.25 g of finely ground fine earth fraction was digested in 0.01 liter of aqua regia (1 HCl:3HNO₃) for one hour, followed by 0.45 µm filtering with an aliquot analyzed using inductively coupled plasma - atomic emission spectrometry. Quality assurance and analytical accuracy involved four certified reference materials and duplicate samples. Soil analysis for the aqua regia digestion was performed by Activation Laboratories (Ancaster, Ontario).

Vegetation from the soil selection areas was performed. Plant sampling was performed mid-summer (July 2026) with selection of fully expanded, mature leaves. Mulberry (Morus spp.) was selected for the Commerce soil site and Giant River Cane (Arundinaria gigantea) was selected for the Caruthersville soil site. Plant material was analyzed for N, P, K, Mg, Ca, S, Na, Fe, Mn, B, Cu, and Zn by Midwest Laboratories (Omaha NE).

Standard macronutrient sufficiency range concentrations (% of total dry leaf weight) for selected trees are nitrogen (2.0% - 3.0%), phosphorus (0.1% - 0.3%), potassium (1.0% - 2.5%), calcium (1.0% - 3.0%), magnesium (0.3% - 0.5%) and sulfur (0.2% - 0.5%) ([16]). These ranges should be regarded as general guidelines, as specific trees do differ.

The dominant soil physical (soil texture and horizon characterization) and chemical properties (pH, soil organic matter, cation exchange capacity) of each soil profile are consistent with the soil’s taxonomy. The Caruthersville pedons exhibit silt loam to sandy loam horizons, slightly alkaline pH levels and limited soil organic matter contents (Table 1). The soil organic matter content is 1.1% in the A horizons, whereas the soil organic matter contents in the deeper horizons irregularly alternate from 0.4 to 1.3%. The Bray-1 phosphorus concentrations are appreciably greater than the 22.5 ppm threshold for a 100% soil fertility index. Calcium is the dominant exchangeable cation, with the exchange capacity varying from 10.0 to 23.3 cmol∙kg⁻¹. The exchange capacity values roughly correspond with the clay content. The unit cmol∙kg⁻¹ is a centimole of protonic charge per kilogram.

The Commerce pedons exhibit silty clay to clay-textured horizons, having slightly alkaline pH levels and moderate soil organic matter contents (Table 2). The soil organic matter content is 3.4% in A horizon, with deeper horizons having somewhat smaller values, ranging from 2.2% to 3.1%. The Bray-1 phosphorus concentrations are substantially greater than the 22.5 ppm threshold for a 100% soil fertility index. Calcium is the dominant exchangeable cation, with the exchange capacity varying from 23.6 to 40.4 cmol∙kg⁻¹.

Table 1.Routine chemical composition of the Caruthersville soil pedons.

Table 2.Routine chemical composition of the Commerce soil pedons.

The aqua regia digestion phosphorus and sulfur concentrations (Table 3) are appreciably greater than the Bray-1 P extraction values (a reagent complex involving HCl and NH₄F), indicating that a considerable portion of the phosphorus recovered by the aqua regia digestion consists of phosphorus less labile than recovered by the Bray-1 P extract. The Commerce pedons exhibited greater quantities of phosphorus and sulfur than the Caruthersville pedons, presumably because of the greater clay contents in the Commerce pedons. Within each pedon, the phosphorus and sulfur concentrations were largely similar with profile depth, supporting an interpretation that the phosphorus and sulfur concentrations were, to some extent, inherited during sedimentation.

Table 3.Aqua regia phosphorus and sulfur concentrations.

The Caruthersville soil horizon phosphorus concentrations exhibit mean values of 42.6 ppm for pedon #1 and 57.2 ppm for pedon #2. The Commerce soil horizon phosphorus concentrations of exhibit mean values of 42.7 ppm for site #1 and 38 ppm for site #2. The mean phosphorus concentrations of the two pedons of the Caruthersville series and the mean phosphorus concentrations of the two pedons of the Commerce series are not significantly different. The mean phosphorus concentrations of the pooled Caruthersville pedons (49.9 ppm) and the pooled Commerce pedons (40.3 ppm) are significantly different (P=0.018).

The Caruthersville soil horizon sulfur concentrations exhibit mean values of 34.3 ppm for pedon #1 and 4.3 ppm for pedon #2 respectively. The Commerce soil horizon sulfur concentrations exhibit mean values of 3.4 ppm for pedon #1 and 3.1 ppm for pedon #2. The mean sulfur concentrations of the two Caruthersville pedons are significantly different (P = 0.0034), whereas the mean sulfur concentrations of the two Commerce pedons are not significantly different.

Plant tissue samples were obtained to complement the soil fertility estimates. Plant tissue concentrations for the Caruthersville and Commerce soil sites are different, presumably because of the vegetation and soil fertility differences (Table 4). For the River Cane samples at the Caruthersville soil site phosphorus, calcium and sulfur were sufficient, whereas nitrogen, potassium and magnesium were slightly deficient. For the Mulberry samples at the Commerce soil site, nitrogen and phosphorus were somewhat above sufficiency, whereas potassium, calcium and sulfur were sufficient and magnesium was slightly deficient. Thus, the selected plants, at both sites, are accumulating nutrients and contributing to the maintenance of ecosystem services. It is noteworthy that both phosphorus and sulfur, for both sites, have plant tissue concentrations that are considered sufficient and not surplus.

Table 4.Plant tissue analysis of vegetation at two soil series sites (% Dry Weight).